1. Field of the Invention
The present invention generally relates to an electronic balance for weighing the mass of an object, in particular, to an electronic balance capable of performing a sensitivity calibration by using internal weights.
2. Description of Related Art
Generally, an electronic balance is configured as follows. A load sensor installed in a load detecting mechanism is used to detect a load resulting from loading an object to be weighed. A digital conversion data generated by digitalizing a detected signal is minutely collected, and limited n data is averaged by, for example, a moving average method. In addition, the averaged data is converted to a mass value in an operation processing by being multiplied with a sensitivity calibration coefficient or a span coefficient, and the mass value as a weighed value is shown on a display or printed out by a printer.
As for the electronic balance, and particularly for a high precision electronic balance, the ratio between a maximum weighed value and a minimum weighed unit value even reaches 1/1,600,000-1/2,000,000 (0.5 PPM). However, electronic parts and other components have stronger temperature dependence, or their electronic or mechanical properties may vary with time. For example, among the components, a permanent magnet has a magnetic field intensity with a temperature dependence of 200 to 400 PPM/° C., and a year-varying property of 10 PPM/month etc. Therefore, various strategies on weighing, such as temperature correction in terms of hardware and software, are adopted, or a precise adjustment is performed during manufacturing. However, a limit of the temperature dependence of the span is controlled in a range of approximately 1 to 2 PPM/° C., even if the ambient temperature variation during weighing is only 1° C. Further, when an object of a weight close to the weighted value is weighed, the reading may have an error of 2-4 counts of a limited value. Therefore, the disadvantage of an electronic balance is that if the electronic balance is not being used in a constant room temperature, it is impossible to perform a reliable weighing.
The electronic balance generally has a calibration function. External weights with known mass or internal weights of the electronic balance for sensitivity calibration are loaded on a load detecting portion to update the sensitivity coefficient for mass conversion, and the calibration is performed to make the weighed display value be consistent with the mass of the weights for calibration. This operation is usually called a sensitivity calibration or a pan calibration. However, the sensitivity calibration should be performed, at any time on demands, to eliminate the sensitivity variation not only during the setting of the electronic balance but also due to year variation of any component in the electronic balance, or performed when a preset temperature variation occurs, or performed periodically in a use state at a preset interval (for example, referring to Japanese Patent Publication No. 2002-214033).
In view of the above, the conventional electronic balance generally has a sensitivity calibration function and is configured as follows. The weighed value is shown on a display or is printed out by a printer. Thereby, through frequent sensitivity calibration, a reliable weighing can be performed. The weighed value is obtained by multiplying a load detecting signal from a load sensor with the calibrated sensitivity coefficient, and then converting the product into a mass value. The load sensor detects the load resulting from loading the object to be weighed. However, as the calibration is performed at any time on demands, for an electronic balance with a sensitivity temperature coefficient of 10 PPM and a maximum weighed value of 300.000 g, in order to control the tolerance resulting from the temperature variation to be smaller than or equal to a minimum weighed unit value, the sensitivity calibration has to be performed each time the temperature varies by 0.33° C. Particularly, even if the user determines that a tolerance at such degree is unnecessary for the weighing precision, the electronic balance may still carry out the automatic sensitivity calibration during the weighing or the key pressing operation, so the weighing or the key pressing operation may be interrupted, and the weighing efficiency decreases.